1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and an image forming apparatus provided the same. More specifically, the present invention relates to the electrophotographic photoreceptor containing in its outermost surface layer fluorine resin fine particles having an average primary particle diameter of 0.1 to 0.5 μm and to the image forming apparatus provided with this photoreceptor.
2. Description of the Related Art
In electrophotographic image forming apparatuses (hereinafter, also referred to as electrophotographic apparatuses) that are used as copying machines, printers, facsimile machines or the like, an image is formed through the following electrophotographic process.
First, a photosensitive layer of an electrophotographic photoreceptor (hereinafter, also referred to as simply “photoreceptor”) included in the apparatus is uniformly charged at a predetermined potential by a charger.
Subsequently, the photosensitive layer is exposed to light such as laser light emitted by exposure means according to image information, thereby forming an electrostatic latent image.
A developer is supplied from developing means to the formed electrostatic latent image; and a component of the developer, that is, colored fine particles referred to as a toner adheres to a surface of the photoreceptor so that the electrostatic latent image is developed and visualized as a toner image.
The formed toner image is transferred from the surface of the photoreceptor onto a transfer material such as recording paper by transfer means and is fixed thereon by fixing means.
Not all the toner on the surface of the photoreceptor is, however, transferred onto the recording paper during the transferring process by the transfer means; and some toner is left on the surface of the photoreceptor. In addition, some paper powder of the recording paper having been in contact with the photoreceptor during the transferring process might adhere to the surface of the photoreceptor and remain thereon.
Such foreign matters as the residual toner and the remaining paper powder on the surface of the photoreceptor cause an adverse effect on quality of an image to be formed and thereby are removed by a cleaner.
In recent years, there have been technological advances toward a cleaner-less system, that is, a developing and cleaning system in which the foreign matters such as the residual toner are removed and collected without using independent cleaning means but using a cleaning function added to the developing means.
In this method, the surface of the photoreceptor is cleaned; then electrical charges on a surface of the photosensitive layer are removed by a discharging device to eliminate the remaining electrostatic latent image.
The electrophotographic photoreceptor used in such an electrophotographic process is constructed to comprise the photosensitive layer that contains a photoconductive material and is stacked on a conductive substrate made of a conductive material.
Used for the electrophotographic photoreceptor is an inorganic photoconductive material or an organic photoconductive material (hereinafter, referred to as an organic photoconductor (OPC)). As a result of recent research and development, organic photoreceptors have improved in sensitivity and durability and thus have been used more commonly today.
In terms of the construction of this electrophotographic photoreceptor, multilayered photoreceptors are very much in the mainstream of photoreceptors recently, in which a photosensitive layer comprises the following functionally-separated layers: a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material. Most of these photoreceptors are negatively chargeable photoreceptors in which a charge transport layer made of a charge transport material having a charge transport ability molecularly dispersed in a binder resin is stacked on a charge generation layer made of a charge generation material vapor-deposited or dispersed in a binder resin.
In addition, monolayer photoreceptors have been proposed, in which a charge generation material and a charge transport material are uniformly dispersed or dissolved in a same binder resin.
Furthermore, in order to improve quality of an image to be printed, an undercoat layer may be provided between the conductive substrate and the photosensitive layer.
A disadvantage of the organic photoreceptor includes surface wear caused by the sliding and brushing of a cleaner or the like on a periphery of the photoreceptor because of the nature of organic materials. In order to overcome this disadvantage, attempts have been made so far to improve mechanical properties of the materials of the surface of the photoreceptor.
There have been known a method such that a protective layer is provided to an outermost surface layer of a photoreceptor so as to give lubricity (see, for example, Japanese Unexamined Patent Publication No. Hei 1(1989)-23259) and a method such that a protective layer contains filler particles (see, for example, Japanese Unexamined Patent Publication No. Hei 1(1989)-172970). In such methods, it has been considered to add fluorine resin fine particles as a filler to the surface (see, for example, Japanese Patent No. 3416310). As one of their characteristics, not only do the fluorine resin fine particles as the filler improve mechanical properties of the photoreceptor, but the fine particles also reduce friction between the photoreceptor and a member coming into contact with the photoreceptor during the process by giving the photoreceptor lubricity owing to a high lubricating function derived from their material; therefore, the fluorine resin fine particles contribute to improvement of printing durability of the surface of the photoreceptor.
Fluorinated fine particles, such as tetrafluoroethylene resin (polytetrafluoroethylene (PTFE)) fine particles, have an excellent lubricating function as a material but are disadvantageous in that these particles have a very large particle-to-particle attraction force and are extremely poor in dispersibility because of a lack of polarity. Accordingly, it is necessary to use a dispersant upon dispersing the tetrafluoroethylene resin fine particles to be used for a photoreceptor (see, for example, Japanese Patent No. 3186010; Japanese Patent No. 5110211; and Japanese Unexamined Patent Publication No. 2009-145480). The dispersant is capable of improving the dispersibility of the tetrafluoroethylene resin fine particles in the photosensitive layer and of preventing deterioration of sensitivity characteristics of the photoreceptor caused by PTFE. The tetrafluoroethylene resin fine particles dispersed uniformly in the photosensitive layer, however, have problems such that the fine particles form trap sites on their surfaces that trap photocarriers having been transferred and that the photoreceptor decreases its sensitivity because of the trapped photocarriers, resulting in a decrease in concentration and image quality.